Half the Step, Twice the Mystery─Kinetically Controlled Spin Crossover in an Anticooperative Pyridine-Coordinate <scp>Jäger</scp>-Type Iron(II) Complex

In the solid state, molecular features of spin crossover (SCO) are superimposed by lattice effects, which often impose cooperative and/or anticooperative effects among the spin centers. As predictable access to cooperative materials is a key requirement for technical utilization of the phenomenon, a molecular understanding of cooperative pathways is required. In this work, we discuss cooperative pathways governing SCO in single crystals and powders of the octahedral complex [FeL(py)2]. The asymmetric Jäger-type ligand H2L (H2L = Methyl-3,4-bis(2-acetyl-3-oxo-1-butenylamino)benzoate) varies the planar directing C2v-symmetric N2O2 coordination motif through incorporation of a remote ester moiety. SQUID magnetometry, 57Fe Mössbauer spectroscopy, powder X-ray and single crystal diffraction studied at variable temperature find SCO in [FeL(py)2] kinetically controlled but incomplete as evidenced by cooling-rate dependent apparent hysteresis widths and scan-rate dependent trapping of metastable high spin species (TTIESST = 77 K). A crystallographic phase transition from orthorhombic to monoclinic goes along with a structural disproportionation toward a 1:1 [HS-LS] lattice at 100 K. Detailed analysis of close contacts in the crystal packing, supplemented by density-functional theory associate both the 1:1 [HS-LS] stoichiometry and the kinetic hindrance with rotational motions of axially coordinated pyridine moieties within the network of H-bonded sheets.